Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, due to a developing shortage, the cost of propylene is likely to increase. Thus, a process that uses higher alkenes instead of propylene as feed and co-produces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenols.
One such process proceeds via cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene (analogous to cumene oxidation) to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts. Although various methods are available for the production of cyclohexylbenzene, a preferred route is disclosed in U.S. Pat. No. 6,037,513, in which the cyclohexylbenzene is produced by hydroalkylating benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product.
However, one or more steps of the process described above can produce substances that are detrimental to process efficiency. In particular, one of the problems associated with this process is that some amount of cyclohexenylbenzene (phenylcyclohexene) is inevitably formed during the oxidation/cleavage step. WO 2011/100013 discloses that cyclohexenylbenzene may be contacted with hydrogen in the presence of a hydrogenation catalyst under hydrogenation conditions to form cyclohexylbenzene (or phenylcyclohexane), which can then be recycled to produce phenol using processes described above. However, the hydrogenation of cyclohexenylbenzene back to cyclohexylbenzene often produces bicyclohexane (BCH) as an undesirable byproduct. BCH is particularly undesirable because BCH has a boiling point very close to that of cyclohexylbenzene, making them difficult to separate by conventional techniques, such as distillation. It is therefore highly desirable to minimize the formation of bicyclohexane (BCH) while hydrogenating cyclohexenylbenzene to cyclohexylbenzene.
Accordingly, there exists a need for highly active, highly selective processes of hydrogenating cyclohexenylbenzene to cyclohexylbenzene, thereby minimizing the production of unwanted byproducts, for example, BCH.